clientmail.sixent.net has no NS records.
NS records for this zone are defined at sixent.net.
NS records for this zone are defined at sixent.net.
Question and response
Question
dig clientmail.sixent.net. NS
Answer
The response has no answer records.
Authority
sixent.net. 900 SOA ns-865.awsdns-44.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0